The answer is 85.00g.
A solution's percent concentration is defined as the mass of solute divided by the total mass of the solution and multiplied by 100%.
c = (mass_(solute))/(mass_(solution)) * 100%, where
mass_(solution) = mass_(solute) + mass_(water)
Knowing that we have 100g of solution, we can determine the mass of the solute by
m_(solute) = (c * m_(solution))/100 = (15.00 * 100.0)/(100%) = 15.00g
Therefore, the mass of water required for this solution is
m_(water) = m_(solution) - m_(solute) = 100.0 - 15.00 = 85.00g
This is a case where it is easy to eyeball the answer just because we are dealing with 100g of solution...so a concentration of 20% would have required 80g of water and 20g of HNO_3, a concentration of 8% would have required 8g of HNO_3 and 92g of water, and so on...
